1. Field of the Invention
This invention relates to a switching voltage regulator having a protective device of a switching element for controlling the output thereof from thermal breakdown.
2. Description of the Prior Art
FIG. 1 shows an exemplified block diagram of a conventional switching regulator generating a stabilized power supply voltage. The regulator in FIG. 1 is supplied with a non-stabilized voltage from an input voltage source 1 on the primary side and provided with a transformer 2 which produces a signal for detecting a primary-side overcurrent, an output transformer 3 which divides the secondary side from the primary side, two diodes 4 which rectify a full-wave current at the secondary side, a choking coil 5 and a capacitor 6 which smooth the secondary-side current, and a MOS transistor 7 which controls a primary-side current by switching. A control circuit 8 detects deviation from a set voltage, and modulates the pulse width of the primary-side current in accordance with the deviation. Thus the control circuit 8 controls the output voltage at the output tereminal by means of a change in the on-off interval of the switching transistor 7. The control circuit 8 also detects an excessively high or low output voltage by way of a control processor 9, and further detects the primary-side overcurrent with the transformer 2, and thereby controls the primary-side current in a similar manner in accordance with the detected results. The control processor 9 receives the output voltage to monitor as well as to keep the switching transistor 7 turn-off. Numeral 10 shows an output terminal.
In this way, the switching-type voltage regulator in FIG. 1 is protected by detecting any abnormal output voltage with the control processor 9 and/or the control circuit 8 to thereby turn off the switching transistor 7 to bring the power supply output to a halt. Hence, ordinary output abnormalcy can be coped with, but there is a problem that the conventional voltage regulator cannot handle satisfactorily thermal runaway of the transistor 7 and/or diodes 4 to be caused when the ambient temperature rises for some reason. The above conventional voltage regulator must detect an abnormal stabilized output due to thermal runaway of the transistor 7 and/or diodes 4 before bringing the output of the voltage regulator to a halt and, therefore, protects the regulator a little too late, resulting in quite possible failure to prevent the transistor 7 and/or diodes 4 from being damaged or destroyed.
Thus, an object of this invention is to provide a a switching-type voltage regulator, capable of securely preventing a component part or parts of the regulator such as a switching transistor and a rectification diode from being damaged or destroyed due to a temperature runaway.